Electronic discriminator



June 10, 1947. w. R. FERRIS 2,422,088

' ELECTRONIC DISCMMINAT'OR Filed April 3, 1944 lid 000, 000

Patented June 10, 1947 ELECTRONIC DISCRIMINATOR Warren R. Ferris, Kingston, N. J., assigno1 to Radio Corporation of America, a corporation of Delaware Application April 3, 1944,*sern1 No. 529,231

11. Claims.

My present invention relates to electronic frequency discriminators, and more particularly to a frequency modulation discriminator of the purely electronic type.

Prior methods of deriving amplitude-variable currents from frequency-variable currents have in the main utilized passive networks and relied upon phase shifting characteristics of reactive elements included in the passive networks. One of the main objects of my present invention is to provide a novel method of frequency discrimination, i. e. translating frequency-variable 'currentsI into amplitude-variable currents, wherein the discriminator i-s an active device comprising lat least two electron beams of relatively diierent time delays thereby providing a net output current whose amplitude is a function of the relative phases of the beams.

Another important object of my present invention is to provide a novel and improved arrangement for deriving amplitude modulated carrier wave currents from angle modulated oarrier waves.

Another objectv of this invention is to provide .an electronic method of phase splitting very high frequency currents; the method basically involving the production of two electron beams, the subjection of the beams to relatively different time delays, and the derivation from these beams of currents of different phases. Basically, my method provides time delay by electron transit time in place of electromagnetic effects.

A more specic object of my present invention is to provide apurely electronic frequency modulation (FM) discriminator employing a device having two steady electron beams varied by the FM voltage, the beams being caused to differ in transit time sotlnat their net effect on a common collector electrode will be zero for alternating .currents at the mean frequency of the FM voltage, whereas for other frequencies the collector electrode current will be the algebraic sum of the alternating'current components of the beams.

Still other objects of my invention are to provide novel circuits for deriving phase-split currents from angle modulated carrier currents of high frequency by controlling two electron beams of different time delays lwith anglemodulated carrier currents, and deriving from secondary emission currents produced by the time-delayed beams the desired phase-split currents.

' The novel features which I believe to be characteristic of my invention are set forth with parvticularity in the appended claims; the invention 2 ganizations whereby my invention may be carried into effect.

In the drawing:

Fig. l shows schematically the networks of an FM receiver adapted for use with an electronic discriminator of my invention, the discriminator device being shown in lateral section,

Fig. 2 graphically shows the ideal characteristic of the discriminator device of Fig. 1,

Fig. 2a represents an oscillogram of the two currents at the 180 degree point of Fig. 2,

Fig. 3 shows a modified form of discriminator device, preferable lat ultra-high frequency,

Fig. 4 illustrates the manner of utilizing secondary emission in an electronic discriminator device of the type shown in Fig. l,

Fig. 5 shows a modification of the device shown Y in Fig. 4, wherein a pair of independent collector circuits are employed to provide currents of split phase.

Referring now to the accompanying drawing, wherein like reference characters in the different figures correspond to similar circuit elements, my novel electronic discriminator device is shown applied rto a systemfor receiving FM signals..

Since those skilled in the art of FM communication are fully acquainted with the manner of constructing an FM receiver system, the latter is shown in purely schematic form. The FM signals are assumed to be located in the 42 to 50 megacycle (mc.) `band and have a maximum deviation range up to kilocycles (kc.) on each side of the transmitter mean or carrier frequency.

My invention is not restricted to any specific frequency range, nor to any particular frequency deviation range. The FM signal energy is collected at collector I which is preferably a dipole, although any well-known source of signals may be used. The modulation on the carrier may be sound, video or code. The converter 2 may be of a well-known form, and is fed with collected FM signal energy and local oscillations. The latter are produced by la local oscillator capable of generating oscillations whose frequency is chosen to combine with the FM signal mean frequency to provide the converter operating output frequency. For reasons to be later explained I prefer to employ the sum frequency of the FM signal frequency and oscillator frequency as the operating frequency at the converter output. Assuming the oscillator is adjustable for a frequency range of 58 to 50' mc., there will be provided FM signal energy at the input terminals of amplifier 4 whose mean frequency is for example mc.

Where the converter 2 and oscillator 3 are concurrently varied over the respective frequency ranges of 42-50 mc., and 58-50 mc., the converter output energy will have a mean frequency of substantially 100 mc. throughout said frequency alicasc ranges. For operation at xed frequencies, oscillator 3 may be crystal-controlled in the usual and well-known manner. vOf course, the conversion .step may produce a difference beat frequency Where the collected signal energy is sufli- .5

' emit electrons from each face thereof.

ciently high in frequency to result in very high'V 1 frequency beat energy. For example; ior'FMw signals at 200 mc., local oscillations of 0 mc'` could Ioe used to produce lbeatjene'rgy whose ymean frequency is 100 mc. Furthermore, for iiXed Vfrel0 quency operation-of the order of 100 mc. the signals could be applied directly tothe .amplifier l without using conversion. It is to be clearly understood that the 100 mc., value is purely illus'- trative, and may readily be higher or lower in l5 magnitude.

After amplification at amplifier d the FM signal energy may be applied to lamplitude limiter li; The latter can be of any well-"known construction, and generally functions .inthe manner of a readily-'saturable amplifier thereby to eliminate, or greatly reduce, amplitude variations in the'FM signal energy applied to the resonant input circuit Soi the discriminatorrdevice. Circuit 6 is tunedgto i90' mcrwhich 'is the predetermined 25 desired mean frequency Fe oi vFM signal energy kto be applied to the discriminator device. It is to b eunderstood that the selective circuits of 4networks 2, 4,- 5 and''are to be given passband characteristics suiiciently Wide to pass thefmaximum frequency deviations of the signal-energy. Before describing in detail the constructional and functional featuresoi the. electronic dis.-

criminator device, itis pointed out that numeral 'I denotes an output circuit tuned to the mean' or 35 Acarrier frequency Fc of .106.mc. Across thistuned output circuit therewillbe developed amplitude modulated` (AM) carrier wave.v energy of' 100 mc. AnyFM effects remaining are purely incidental,

' and-.do not afectftheoperationof a conventional 40 Y detector. The amplitude modulation will correspend to,and be. representative of, the frequency deviations .ofA the. FM -signalenergy collected Vat collector I- The carrier Wave energy at outputcircuit I will be.y applied to any desired form 45 orzrectiiier. device, such as a simple diode. The

'rectified modulation. may then be amplified in any Well known Vmanner and utilized.. Of course, the electronic. discriminator about lto be described can'bewusedfwith direct.application of frequency, 50 or phase,V variable energy to input circuit 6.

`Also', the generic Y(expressionangle modulated employed hereinisto be understood as including frequency modulation.,V phase. modulation. and

- other related forms of modulation.. My invention 55 lltype.. The type may follow the construction set forth by l-I'.` M; Wagner inhis lll'. S. Patent No. 2,293,413 granted August 18, 1942.. Since the latter patent vgives precise and detailed infor- '65 mation as .to `the manner of constructing tubes .ofthe orbital beamtype, it is believed suicient forthe purposesv of this invention -to .show the general relations of the tube electrodes in the cross-sectional manner used herein. Theftube Venvelope 8 is `provided. with concentric cylindrical: metallicffocusing.electrodes Sand It). The

t 'outerelectro'de' Sis preferably connected to a potential point V.which is Yrelatively negative. with respect to ground; "The inner cylindricalelecand trode I is connected to a source of relatively high positive potential. l

The two cylindrical velectrodeshavea common axis. The electron emitter or cathode, indicated .by numeral I I, is grounded and is constructed to The cathode -II is indicated as hollow and having a ,rectangular cross-section. The heater element is located within the-walls of the emitter. `The cathode element Aisfequidstantly spaced between concentric focusing electrodes 9 and Ill, and providesV a pair'of oppositely directedelectron beams from its opposed flat emission faces. 'Ihe beams pass lthrough the mesh of the control grid i2 Which surrounds the cathode with its Walls parallelvthereto and spaced substantially` uniformly therefrom. f

The control gridlZ is connected :to the. high alternating potential side .of Vthe input circuiti. The 10W potential side. of circuitris. connected to a negative. potential point. approximately the same negative potential as that. ofelectrode ,19; The-flat collectorelectrode I'is locatedinspaced relationto grid 12,. and is inclined .relative there.-

whichis relativelymore positive than the iocusg ingelectrode Ita. It is `to be understcodthat a common direct current vsource maybe used to supply the negative and positive voltagesfor the electrodes with-in the envelope 8. The actual values of plus and Aminus potentials employed may in the light-of the description herein beleit to the discretion of those skilled in the art.

Thev orbital beams emitted in opposite `directions are denoted by dotted circular linesv I4 and I5. The. electronV beam I4 is the long path beam, and flows'. from cathode II through grid I2 in aclockwise direction to the righthand face Y of collector electrode i3. The .beam I5 is the short path beam, andItraVels but a relatively short distance romcathode Il through grid I2 in a counter-clockwise direction to the lefth'and `face of electrode I3. The dotted arcuate lines I4 I5 schematically represent' the electron beams flowing to electrode i3; Thebeam I4 is caused kto follow Va .constantly curved path `in a clockwise directionby virtue of the electrostatic vforce exerted by the positive inner focusing electrode Il); The highlyY positive collector electrodev I Y3jprovide's acceleration forthe electrons.

In order to explain the functioning of theelec'- tronic discriminator tube, there Ywill be iirst set 'forth certain facts-relative tothe electron beams I4 and I5, Itis known that a 4progressive change oi"- phase is suiered by' the alternatinglconvection current component ot-an electron beamY as its constituent electronstraverse a given path. So longas the electronsin the beam keep in step, this isthe only effect of a relatively longspath 'upon-.the alternating convection current component.

The term convection current" is peculiar to electronics, and.` the. current; representing the actual quantity of '.electricity, usuallyV carried'by electrons,` entering a conductor.` The symbolic representation 4di the convection current. is,=pv, Where p is. the chargedensity. (say. incoulombs per cubic centimeter), c is the velocity Vincentimeters Yper second, and i isthe vcurrent density rin amperesY personale-centimeter. The disn vbeing an odd integer.

electron path length in any beam tube is reached when the effect of the initial velocity (or second- 'placement curren is the current induced in a real or hypothetical electrode at a given location by the change of potential gradient on the surface of the conductor with time. Whenever modu- `lated electron clouds r streams are encountered Vcurrents of both above kinds are found in all conductors capturing electrons, but only displacelment currents exist in negative electrodes capturing no electrons. 'Ihe displacement' current associated with the electronic charges is small at low frequencies, but may become very large at ultra-high frequencies. In the present invention displacement current is disregarded, and is readily eliminated by assuming an electrostatic screen is placed around collector I3, as indicated in Fig. 3. It is to be then understood that only convection current contributes substantially to the net output current.

e If two steady electron beams are varied by the same alternating control voltage and subsequently collected by a single collector element, their instantaneous algebraic sum determines the net alternating current flowing in the collector element circuit. Further, if the electrons in two streams having the same alternating current cornponent are caused to differ in transit time by an odd number of half periods, the net effect of the two streams will be to produce in the collector circuit only direct current and no alternating current. If now both streams are varied by an alternating voltage of a somewhat different frequency from that producing zero alternating current in the collector element or electrode, the algebraic sum of the two alternating components will no longer always be zero. The sum of the components will have a steady alternating current component proportional in some fashion to the difference in transit angle resulting from the fact that the transit angles no longer differ by exactly an odd number of half periods.

The discriminator tube has its parameters so chosen that oneelectron beam would arrive at the collector electrode I3 after approximately zero half periods, and the other beam would arrive after as large an odd number of half periods as possible. Thereby, the arrangement is made sensitive to small frequency deviations ofthe signals from the mean or carrier frequency. It is now seen `why it is desirable to operate the tube at a very high frequency, of the order of 100 mc. The effect sought after is best secured at such frequencies unless the dimensions of the tube are made excessively large, since transit angles of even one-half period require a fairly long path unless the control frequency is very high. On the other hand, if the control frequency is made exceedingly high (say for a wavelength of the order of 1 centimeter) it becomes difcult to keep even the short path less than one-half period.

Hence, I propose to permit the short path I to go where it will, and merely lengthen the path I4 by an amount differing from that of the path `I 5 by an odd number of half periods. For example, if the short path transit time is period, the long path transit time would be l 2 ioorz The limit to the useful ary emission scattering) of the electrons inthe beam smears the collected beam out in time phase 'so that its-original modulation'is seriouslydecreased. This requires'several periods in a well constructed tube. So far as actual dimensions and proportions of the discriminator tube` are concerned, a tube suitable for use in a system adapted to receive signals at Ll0 mc., would be little more than one-half inch in diameter and about an inch to one and one-half inches long.

In Fig. 2a`I have shown in purely graphical manner, and by Way of explanationonly,rthe phase relations existing between the currents reachingv the output electrode I3 from the short path beam I5 and the long path beam I4 respectively. The curves a and b of Fig. 2a represent an oscillogram of the two currents at the 180 point of Fig. 2. It will be observed that the short path current, represented by solid line curve a, goes through one complete cycle in one-one hundred millionth of a second at mc. The electron current of the long path, represented by broken line curve b, starts inelectrode I3 vonetWo hundred millionth of a second after the short path current reaches that electrode. The net current collected at electrode I3 depends on'the relative amplitudes and phases of the two beams at the instant of arrival. So long as the principles of the invention are not departed from, electrode I3 can be variously located to suit the desires of the designer.

Fig. 2 relates the net output current". as ordinates in the collector circuit to relative phases of electron beams as abscissae. It -Will be observed that the net output current lthrough electrode ,I3 is zero at 180. Assuming thatA the predetermined frequency Fc of circuit 6 is `such that thenet output current will be somewhat as shown in Fig. 2 for the instant that the signal `energy has a frequency of Fc, it can readily bev-recognized that the slope c of the characteristic of Fig. 2 permits discrimination. As the applied signal energy devlates from Fc `the net output'current flowing in circuit I will vary by virtue of the variable relation in the phases of the alternating components of electron beams I4 and I5. The variable net output current is an amplitude modulated wave whose effective frequency is, in the example given, 100 mc., and Whose amplitude modulation corresponds to the frequency modulation on the original FM Wave. Operation at the zero point on the characteristic c of Fig. 2 may be used for telegraph code. The location of collector electrode I3 to secure the mean net current at Fc may be determined by mathematical calculation to get about periods between the transit times. Also by trial and error in adjusting the overall voltage, there can be determined the location of electrode I3. In practice it would probably be convenient` to make the short transit time of about If desired, and as shown in Fig. 3; van electrostatic screen such as a grounded suppressor grid I5 can be employed around thecollector I3. The

from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In a frequency discriminating system, means for providing a pair of independent electron beams, means for producing a predetermined relative time delay between said beams, an electron collector electrode common to said beams, means adapted to vary the intensity of said beams in response to frequency variations of alternating voltage relative to a predetermined mean frequency, an output circuit connected to said collector electrode tuned to said mean frequency, and said relative time delay being so chosen that a net current flow of predetermined amplitude occurs in said output circuit at said mean frequency whereas the net current flow varies in magnitude in response to said frequency varying voltage.

2. A method of deriving an amplitude-variable voltage from a frequency-variable voltage which includes producing a first beam of electrons, modulating the beam with said frequencyvariable voltage, producing a second beam of electrons, modulating the second beam with the frequency-variable voltage, producing a predetermined relative time delay between said beams, and collecting the modulated beams to provide a net current whose amplitude varies in accordance with said frequency varying voltage.

3. A method of deriving an amplitude-variable voltage from afrequency-variable voltage which includes producing a first beam of electrons, modulating the beam with said frequency-variable voltage, producing a second beam of electrons, modulating the second beam with the frequencyvariable voltage, producing a predetermined relative time delay between said beams, and collecting secondary electrons produced by each of the modulated beams to provide a net current whose amplitude varies in accordance with said frequency varying voltage.

4. A method of dividing an angle modulated voltage into a pair of angle modulated voltages of relatively different phases, which includes producing a pair of electron beams of different time delays, modulating the beams with said original voltage, deriving a separate stream of secondary electrons in response to each beam, and separately collecting said secondary electron streams to provide said pair of voltages,

5. A method of receiving frequency modulated voltages of high frequency, which includes converting the voltages to a higher frequency of the order of 100 megacycles providing a pair of electron beams of relatively different transit times, collecting the pair of beams to provide a net current whose amplitude is dependent on the relative phases of said beams, and modulating said beams with said higher frequency voltages.

6. A method of imparting amplitude modulation to a frequency modulated signal which includes producing two electron beams, frequency modulating the beams, collecting the beams at a common collector, delaying the arrival of one of these beams so as to al-gebraically add the amplitudes of the beam currents thereby to obtain an output voltage having said amplitude modulation.

7. In combination with a source of angle modulated wave energy, a tube having means to provide two separate electron beams of different 10 length, a, common collector for collecting the beams in a predetermined phase difference, and a control electrode for said beams responsive to said modulated wave energy.

8. In a frequency discriminating system, means for providing a pair of independent orbital electron beams, means for producing a predetermined relative time delay between said beams, an electron collector electrode common to said beams, means adapted to vary the intensity of said beams in response to frequency variations of alternating voltage, an output circuit connected to said collector electrode, and said relative time delay being so chosen that a predetermined net current flow occurs in said output circuit at a mean frequency whereas the net current iioW varies in magnitude in response to said frequency varying voltage.

9. In a system for deriving an amplitude-variable voltage from a frequency-variable voltage which includes means for producing a rst beam of electrons, means for modulating the beam with said frequency-variable voltage, means for producing a second beam of electrons, means for modulating the second beam with th'e frequencyvariable voltage, means for producing a predetermined relative time delay between said beams, and a common collector element for collecting the modulated beams to provide a net current whose amplitude varies in accordance with said frequency varying voltage.

10. A system for deriving an amplitude-variable voltage from a. frequency-variable voltage which includes a tube having an emitter producing a first beam of electrons, an electrode for modulating the beam with said frequency-variable voltage, said emitter producing a second beam of electrons, said electrode modulating the second beam with the frequency-variable voltage, means for producing a, predetermined relative time delay between said beams, and separate collector electrodes for collecting separate beams of secondary electrons produced by each of the modulated beams.

11. A system of dividing an angle modulated voltage into a, pair of angle modulated voltages of relatively different phase, whichi includes a tube having an emitter for producing a pair of electron beams of different time delays, means for modulating the beams with said original voltage, means producing a separate stream of secondary electrons in response to each beam, and separate collectors for collecting said secondary electron beams to provide said pair of voltages.

WARREN R. FERRIS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,293,418 Wagner Aug. 18, 1942 2,173,267 Strutt et al Sept. 19, 1939 2,146,607 Overbeek Feb. 7, 1939 2,159,774 Veenemans et al. May 23, 1939 1,952,493 Edelmann Mar. 27, 1934 2,307,693 Linder Jan. 5, 1943 2,284,829 Ludi June 2, 1942 2,289,319 Strobel July 7, 1942 

